AdditionalInformation

TitleSendmail throws error if the mail server for the domain is an external server

SummaryWhen trying to relay mail from the server to a domain on the server which has hosted mail on some other mail server, the sendmail server throws up "User Unknown" error.

Prevention

Details

This problem has been noticed for sites with external email servers and sendmail disabled. The problem exists because of checks introduced in sendmail to give an outright "User Unknown" error message when trying to deliver to a non existing user on a domain. The previous behaviour was to first accept the mail then do a check locally and bounce back with a user does not exist mail if the user did not exist in the domain. The change was made to prevent unnecessary mails in the system which happened when spammers tried to guess usernames on sites using brute force.

This however does not take into consideration the situation where the mail is hosted on another server.

To get past this, Edit the site and enable sendmail on it. Then make sure you have a catch-all account set for the domain. Edit the site to disable sendmail on the server. Make sure that the domain aliases file /home/virtual/domain.com/etc/aliases.db exists after you have disabled sendmail for the site.